The present invention relates to a method for checking the functional capability of a nitrogen oxide sensor. Such a method is known, for example, from DE 103 12 732 B4. With regard to its device aspects, the invention relates to a control unit.
Nitrogen oxide sensors are known, for example, from document SAE 960334 “Thick Film ZrO2 NOx Sensor”. Such sensors have a first chamber and a second chamber, wherein the first chamber is connected to a measuring gas space via a first diffusion barrier. A diffusion barrier is here, for example, a porous structure which impedes a diffusion of gas particles without, however, completely blocking them. The measuring gas is, for example, the exhaust gas of an internal combustion engine. The first chamber has a first oxygen pump cell which is electrically connected to a control unit. The second chamber is connected to the first chamber via a second diffusion barrier and has a second pump cell which is electrically connected to the control unit. In a normal operating mode of the nitrogen oxide sensor, an oxide concentration level in the first chamber is reduced to a predetermined first value with the first pump cell. An oxygen concentration level which occurs in the second chamber is reduced to a second value with the second pump cell by releasing oxygen from the nitrogen oxides, said second value being lower than the first value. The pump flow which occurs here in the solid electrolyte of the second pump cell and therefore in the second pump cell is essentially carried by oxygen ions which result from the decomposition of nitrogen oxides. This pump flow is therefore a measure of the nitrogen oxide concentration in the exhaust gas.